Spark plug and process for making the same



Jan. 31, 1961 N. F. MEREDITH ETAI. 2,969,582

SPARK PLUG AND PROCESS FOR MAKING THE SAME Filed Jan. 19, 1955 INVENTOR SPARK PLUG AND PROCESS FoR MAKING 1HE SAME eil F. Meredith, Cleveland, Ohio, and Karl Schwartz# walder, Holly, Mich., assgnors -to General Motors Corporation, Detroit, Mich., a corporationof Delaware l VFil'ed'JarL 19, 1955', Ser. o. SS`

3 Claims. (Cl. Z9-25.12)

This inventionV relatesto' spark plugs and tothe method for making same. More particularly,` the inventionl relates -to spark plugs of the low voltagev type-which plugs are commonly referred to as low tension or creep gap spark plugs.

One of the goalsof internal combustion ignitionV -re-l search is a spark plug which will give an extremely hot spark and whichwill atv the same time be both reliable andf havega relatively long life. One of the chronic problems of ecient spark plug operation isV that ofl plug fouling as a result of thevcombustion process taking place within the cylinder. Conventional type spark plugs utilize a relatively large air gap in combination with a'- high Yvoltage ignition system. Such systems lare sus'- ceptible to interferenceY as a result of the plug fouling; Recently, however, it has beenV proposed to use a spark plug having a relatively small discharge gapl and being so constructed as to provide a low tension discharge patti for use i'n' a low voltageignition system. By using high energies, a hot spark may be obtained suchl as will inhibit' the tendency toward plug fouling normally found rerice objects are ttaidgin .accordance with duiivention by providing a spark plug having' a smooth, refractory, substantially' .noti-porousy semi-conducting surface on the'insulator abutting the spark gap and in adherencef to thefelectrode surfaces which it co'ntacts',`thev surface `being formed by subjecting the smilconducting composition to high heat and pressure to bring about thefusion andconipression thereof. v l l v I Ather 4o fjects .and advantagesrof, `our invention will be apparent from the hereinafterfollowing description and im the drawing in whiclrl,ligurell ispa vertical vcros'si fro sectional of neemt-coment or the spark plug or oui` inventiontandY Figure y2 is a" horizontal cross-sectional view taken on line Z-'2'of Figure 1'.' v

' Referring'A now tro-.Figure 1 there is shown a spark includes a shell 3 havingv a shield sleeve 5 secured with Y the'upper portion thereof, land a central wire subassenibly'7 positioned concentrically within the Shen 3 and-thesnieidds. t

Thev subassembly 7 comprises a center electrode 9 positioned in' and extending lthrough both ends" of a longitudinally extendihgi'bore in' insulator 11'. The insulator 11 is providedlwith an upper portion" 13 extending concentrically Within' the Shield s'leevegS; a lohver ,portion 15 extending within' the'outer Shelli 3 a'n'dlnk intermediate portion 17 of larger diameter tha'nboth: the: upperl fand lower portions thereby forming an upper shoulder 19 i'r' conventional' spark plugs. Likewise, by reason of the use of a low system voltage, any lfouling which does occur does not have-as harmful an effect as that occurring withl conventional' high voltage plugs.v These low teni sion` orfc'reep gap' spark plugs comprise ,a pair of elect'rodes` separated by an insulator body having the surface' abutting -the spark gap soconditioned a'svto form a path between the electrodes for permitting a low voltage discharge.v Such conditioning has been disclosed yasconsi'stinglo'f a'coating'or layer of semi-conductive material which is sinter'ed or otherwise attached to the insulator surfaceiad as being formed of such materials as metals, met'al oxides, metal carbides, carbon, and various mixtiire'sof oneor more of these compounds with ceramic materiau. t p Y H'o'we'ver; ithas'beenfoun'd in practice that such low tension spark plugs become inoperative after. a veryshort period ofi operation; Itv has been foundf that theshort li'eof'the plugs' is' dueto thewear' of the semi-con ductiv'e'co'atings by reason of the fact` that the coatings were'ifb'undto'be'relativelyporous and subject to erosion vention is tofpro'vide*a-novelmethodforfrnak'ing low teusionsparkfplugs. t 4 I and?" a lower shoulder 21` thereon'. The center elect'rcde V9 is provided,withl a top portion extending into the-shield sleeve 5 and beingladaptedfor eleotricalcon# nection' with the ignitioncable (not shown) of the kSYS- trein4 and with a bottom'- portion or head 23 adjacehtthe bottom: surface 25 of the insulator 11. The insulator 11 andjthe" lelectrode 9 are positioned within anvinner shell' 2.7- which" isof a shape/'generally ycorresponding to that of thelower portion 15, theV insulator being' seated in sealed relaticnshipvon the upper andl outwardly' exi tending portion v29 of the inner shell.v A relatively soft heat'- resistant metal washer 31', i.e. nick'ehunickel alloy; is interposedbetween the upper portion 29 of shell 27 and thev shoulderzl ofthey insulator and serves to pre# Acl'ud'erundue localized stress' upon the insulator andas a ysealing member. T he lower portion`33 of the shell 27 4extend/s downwardly and'inwardly beyond thewbottom surface 4 25ct the insulator to apoint adjlacentrthe c en ter electrode'he'ad 23 thus4 serving asfthe 'ground orroute electrode o'fthe's'park plug 1.1 As is; clearlyshown'in Figure" l, a'lse'mi-conductor body 35 is provided on the bottom surface 12 of theinsulator and between the center electrode 9`an'd the ground electrode 33 to'prese'nt a? surface portion thereof in abutment with the spark gap formedbetween the electrodes. Thesemi-conductor body is adhered tothe metal electrodes andmay be fsedto theinsulat'or as is described more fully hereinafter andv isthereby provided lwith ample-heat conducting surfacesv for preventing undue heatingy and softening thereof. 4 n

The innershell 27 is' supportedA concentrically within the outer shell 3 bymean's of'its upper portion 29`which is seated on aledge` 37 formed onthe innersurfaceof the'outer'shell 3,' a nickel'sealing gasket39f'b`eing tioned therebetween.v t In yorder to seal' thesubasse 7 within the outerV shell, asoft metal-gasket 41A@ copper; is positioned on the upper shoulder-19d theinf sulator 1-14 and the shield 'sleeve' 5 is7provide`d externally threaded portionon its4V lt'wve"end--for cti-fy operation with an internally? threadedpdrtii oh the upper-parrotthe *shell" 3.` Inthisninne'r'i' y'efisleve 5 may be threadedetighuy intofthe shelts `c` relationship within" the" shells" 3l insulator 11 in sealed K and-27. The jam-:nut f 45 'threaded-onf th'e uppereportio of the center electrode 9 and bearingon theto'pfof init' sulator fit1 serves to keep the insulator in assembled po sition and to rigidify the subassembly 7.

As shown on the drawing, the plug 1 is adapted to be mounted on the engine by means of a mounting pad 43 secured to the shell 3 by any suitable means such as welding. In order to assist in removing heat from the subassembly 7, the shell 3 may be provided with one or more apertures for passing cooling air into the annular space Ibetween the yinner and outer shells and over the spark'gap area.

The insulator 11 which is positioned adjacent the spark gap should be formed of a material having high heat resistance in order to withstand the high temperatures Y encountered in normal operation. Though other materials may be used, it is preferred to use a sntered aluminatype insulator because of its high mechanical strength, high heat resistance and high fusing temperature.

It is to be understood that the specific structure of the spark plug shown and described herein is given merely for purposes of illustration and that numerous other embodiments and modifications are possible and are within the full and intended scope of the invention. The only characteristics of the spark plug structure are that it be provided with an insulator having a surface portion positioned between the electrodes and adjacent the spark gap so as to provide a dense, refractory, substantially nonporous semi-conducting surface abutting the spark gap as a path for the electrical discharge, the surface portion being adhered to the electrode surfaces.

We have discovered that the improved low tension spark plug of our invention may be formed by utilizing a semiconductor composition which exhibits properties of thermoplastcity on being heated to a temperature of from about 1500 F. to about 2200 F. and which hardens to a relatively non-porous, refractory material on cooling. The resultant composition should be resistant to corrosive effects of the combustion gases and to the wear effects of the extremely high sparking temperatures and energies.

We have found that compositions comprising semiconductor materials such as the stanno-titanate compositions described in copending application S.N. 357,906 filed May 27, 19'53, various carbides as silicon carbide,

germanium, various suldes and such conductor materials as copper, nickel, etc., in admixture with glass exhibit the desired properties on being hot-pressed to the fusion point of the glass followed by cooling. While we prefer the use of glass as the constituent imparting properties of thermoplasticity, it should be understood that other materials such as calcium fluoride may be used.

Our preferred compositions are those comprising admixtures of the stanno-titanate semi-conductor materials in glass not only for the reason that hot-pressing produces the desired physical characteristics but also since these vcompositions may be readily controlled as to resistance value by control of the amount of reducing agent, i.e., aluminum, carbon, contained therein and since they exhibit stabilized electrical characteristics such as the voltage and temperature coeiiicients of resistivity. Thus, the use of a glass base stanno-titanate semi-conductor composition for forming the bottom surface portion of the insu lator results in a low tension plug having relatively long life and stable electrical characteristics.

As is disclosed in copending application S.N. 406,338, tiled January 26, 1954, various types of glass may be used such as barium-berate, magnesium-berate, and boroaluminum-silicate or alkali-boro-silicate type glass, an example of the latter being the commercially available Corning #774 glass. We have found that the barium borate and magnesium-berate type glasses are too soft for use alone as the hot plastic and bonding ingredient in our semi-conductor compositions. This undesirable characteristic may be overcome by mixing the soft glass with such hard glass as the alkali-boro-silicate type glass. Examples of the types of glass referred to are shown in the following table: n

Percent Si02 79.12 R203(A'1203Fe203) MgO 0.29 CaO 0.17 Nano 3.72 B203 14.40 Ignition loss 0.18

Magnesium-barata glass B203 95 MgO 5 Barum-borate glass B203 BaOg 25 The range compositions of stanno-titanate semi-conductor material found best suited for our purpose is as follows:

It should be noted that vanadium pentoxide may be used in place of Ta205 and that tungsten oxide may be used in place of M003. Likewise, such materials as magnesia, mullite, Zircon, chrome oxide, etc. may be used in place of A1203.

The semi-conductor material in accordance with this invention may be prepared, after weighing out the desired amounts, by thoroughly dry mixing in a Lancaster mill, the constituents being of such size as to pass a 325 mesh screen (43 microns) with most particles being less than l0 microns. The batch is then placed in a suitable container and calcined at a temperature of around 1400* C. in an atmosphere that may be slightly oxidizing, though this is not essentialr and we do not wish to be limited thereto. For example, a reducing atmosphere may be used thereby giving further control of the resistivityof the material. The calcined material is then ground to the desired state of subdivision, preferably ner than 200 mesh. If desired, the material may be briquetted before firin Tle addition in very small quantity of such reducing agents as powdered aluminum and carbon, the latter hav ing a particle size of about 0.3 micron and being available commercially as Thermax, enables almost precision-like control of the resistance of the final product, the resistance being lowered as the amount of reducer is increased. The amount of reducer added should be so small asto exist in the product as a discontinuous phase and function not as a conductor material but solely as a reducing agent. Though the exact nature of the interaction of the materials in the composition is not known, it is theorized that the glass phase acts to form a multitude of reaction bombs each containing reducer and semi-conductor material which react in the course of hot-pressing the desired element to form a glass-like, vitried, semi-conducting structure. The reaction in the glassphase apparently also results in the materials being integrated therewith.

It has been noted that the aluminum has a greater effect in reducing the resistance than hastthe carbon and .annessa areY obtained. by using `quantities shouldbe. understood .thatthis 15.11912 plications is as follows:

Glass Semi-conductor material',

The semi-conductor composition may be prepared in granular form byl rst dry mixing the materials and then adding water to Amake -a plasticyemass. The plastic mass -is then forced through a 20 mesh screen `and the` resulting granules dried. VThe driedmateiahisthen regranulated through a 28 mesh screen and the material retained between 28 and 100 mesh is used. This sizing has been found to produce granules which are most suitable for uniform volumetric feed. Alternatively, the materials may be dry mixed and formed into a free-flowing slip by addition of water. The slip is then passed into a spraydrying tower where the desired agglomerates are formed.

In the hot-pressing operation the semi-conductor composition is heated to the temperature necessary to develop the plastic flow characteristics of the glass phase. Various types of glass require different temperatures and we have found that a temperature of 1550 F. to 1900 F. suffices for most glasses. Likewise, a pressure suflcient to cause the glass to flow into the mold contour and produce a relatively non-porous structure is applied, the composition being kept under pressure while cooling until the glass has become rigid. We have found that a pressure of 300 p.s.i. or greater is suitable.

The fluidity of the final composition, as exhibited during the hot-pressing operation, is controlled by the presence of a filler or diluent material which does not react chemically with the other constituents of the composition. We have been a-ble to obtain very satisfactory results with a -48 to +100 mesh mullite though other materials such as borolon, Zircon, chromium oxide and aluminum oxide, etc. may be used. The filler or diluent material may be added to the semi-conductor composition or may be only that amount present in the semi-conductor (stanno-titanate) material. It has been noted that the filler also increases the temperature resistance of the formed element after hot-pressing, the composition becoming more refractory and less uid than on the first heating.

Since the glass phase semi-conductor composition is best handled in a granulated form, a binder such as bentonite, a very plastic aluminum silicate, is added to bond the particles together during processing.

In forming the low tension spark plug of the type shown on the drawing, a suicient quantity of the semi-conductor composition, for example, any of the glass base stannotitanate compositions described, is cold-pressed in a mold to produce a preform of the desired size and shape. The preform used to hot-press the semi-conductor body 35 shown in Figure l was prepared by pressing about 0.30 gm. of the semi-conductor composition, in powder form, in a steel mold at pressures of the order of 1000 p.s.i. or greater. An annular preform ring about 1A" high by wide by 1,6" thick was thus obtained.

preform is placed .around .the-.center .and .thermen .shell .2.7, the. outer electro.

:into .nasi on, gthetthreeipieces .beine PQ pressing die. .5 onto the preforrnso. asnot trnszracli or, deform'it. y

'The assembly'is then heated to bring the temperatlllne up t thatsuiii' nt tddcvelopghegplastic 4flow characteris- A gia `11ans, and; is held ,atsueh temperature 'for "..period 'amena-,tpressureasuicient ,to compress .the Plame', m. .S S..,i.1tia.d.lls., HQIIPQfovlS-lS-tmitue being thenpappled 'by nomination, ,of torce .to .the `tot! .Qtthe ,insulator`V 1 1. Y In.t11`e'..cas.e `fj an alkali-.boro-silicate.glass lencinas Cor` been,foundkthataempera- ,F- at. a Pressure..of about 1t should libe.` noted that tion, thefinsulator itself a the ,4 otgpressineoneration- .In this, ,nerthas or,massformsa.monolithic .Structure with'the -insulator 11 and adheres rmly t0 Jthe metalY electrodesiassembly 'is .then cooled `u der vpressure 'to a temperature Aof about f1.00.0 F. at which "fpinttherigiid.Sabas mbly may be removed fromthedie V4audgis reads/fior Yassembly 4within .the outer lshell ,av in the aforesaid ,manner- Weihavefnund ,thaty thetime .at .which thenreformf 4tis :hel'd at theplasticizing temperature has edirect. inuenee On the 'HOW 'charatsistesl 1f this une .or ".sakiegme riod is prolong'edtbeyond a certain point, the glass may react sufficiently with some of the ingredients of the semiconductor material to cause the glass phase to become more refractory, that is, more viscous with a decrease in ow characteristics. This property is desirable since the final semi-conductive body then requires a higher temperature before any deformation or softening is encountered. A soaking period of from 1 to 15 minutes in most cases yields the desired results, a period of from 5 to l0 minutes having been found necessary in the case of Corning #774 glass. If the soaking period is too long, the semi-conductive body becomes too refractory to compress into place.

The atmospheric conditions in the furnace are of importance in that a slightly oxidizing condition gives greater electrical consistency than that obtained when using a controlled atmosphere such as nitrogen. In the latter case, an enclosed bomb is used during the hot-pressing operation.

We have likewise found it necessary to have a separator of a non-reactive material between the insulator 11 and the semi-conductor preform in order to prevent cracking of the insulator tip on cooling. A small preform disk of talcum powder inserted between the two elements has been found suitable. However, the talcum does not permit a monolithic assembly between the insulator and the preform and the subasscmbly is then not as rigid as it' might be. We have found that by substituting fluorspar for the talcum a very rigid assembly can be obtained without causing the insulator tip to crack.

As has been described above, We obtain a hard, smooth, non-porous semi-conductor surface on the spark plug insulator by using a semi-conductor material in a glass phase. Though our preferred embodiment as described is a glass-filler-semi-conductor material-reducing agent composition, other semi-conductor materials and/ or conducting metals may be used with glass or other material having properties of thermoplasticity as described. The glass phase serves as the hot plastic and bonding ingredient. It permits the semi-conductor composition to be hot-pressed into place and to bond to the ceramic and metal parts and form a semi-conductor surface portion abutting the spark gap which has the desired physical and electrical properties. Since the semi-conductor composition adheres to the metal electrodes and may be fused to the bottom surface of the insulator, greatly improved heat conductivity is obtained with resultant longer life 75 for the plug. A

'Serves the du i u It vshould be understood that though we have described four invention in terms of a particular structure, various modifications will be Vapparentjfromrthe above .description and it isintended that such variations be within the scope of our invention as defined by the claims which follow.

g We claim:-

'positioning said preform about said center electrode, as-

sembling said" outer electrode about said 'preform Yand said center electrode, positioning said electrodes with a predetermined gap between'their ends, said preform being in contact withthe surface of said electrodes and abutting l'said gap, positioning a separator disk on the upper sur- [faceI of said preform, positioning said insulator about said center electrode and resting on said disk, applying heat to said preform to raise the temperature thereof to the softening point and applying pressure on said insulator to compress the preform to a dense substantially non-porous mass, and cooling said composition lwhile maintaining the application of pressure until said composition hardens, said composition being adherent to said electrodes to form a good heat conducting path therebetween.

i' 2.'Th'e process as set forth in claim 1 wherein said composition is held at the softening temperature for a ,small but suflicient period of time to permit said compo- ;sit'ioin to become m'ore refractory.

3. The 'process as set forth'in claim 2 wherein the temperature and pressure are maintained, respectively, at Vabout 1700 to` 1850 F. and upwards of about 300 p.s.i., ,the time period being from 5 to 10 minutes and the separator disk being formed of uorspa'r.`

References cited in the me of this patent UNITED STATES PATENTS' Sullivan Oct. 15, 1918 1,363,843 Crowther VyDec.'28, 1920 2,064,089 Stemmeler Dec.l 15,1936 2,360,287 Smith Oct. 10, 1944 2,377,481 Christie June 5, 1945 2,578,754 Smits Dec. 18, 1951 2,592,754 Smits Apr. 15, 1952 2,699,158 Purdy et al. Jan. 11, 1955 2,712,685 Johnson et al. July 12, 1955 2,717,438 Schwartzwalder et a1. Sept. 13, 1955 FOREIGN PATENTS Australia June 12, 1952 

